1. Field of the Invention
This invention relates to a small-sized plunger and more particularly to a small-sized plunger which is employed for aperture control or shutter control of a camera, tape rotation direction control or start/stop control of an audio device or VTR, or a device requiring a power source such as a household electric appliance and others, and has a structure in which a part of a yoke is passed through a coil bobbin.
2. Description of the Related Art
A previously known plunger is a self-holding type plunger in which with a movable piece is attracted to a yoke by a permanent magnet, an opposite magnetic field is generated by passing a pulse current through a coil to cancel the attractive force in the yoke so that the piece attracted is released from the yoke. This plunger has a structure as shown in FIG. 12.
A plunger 100 shown in FIG. 12 includes a yoke having a pair of first legs 101A, 101B and a coupling 101C is formed of a magnetic material such as Fe (iron) in a ⊃-shape; a permanent magnet 102 is mounted in a slit 106 formed by a gap between the first legs 101A and 101B of the yoke 101; coil 104A, 104B are wound on a pair of cylindrical coil bobbins 103A, 103B, respectively; and a movable piece 105 having a pair of second legs 105A, 105B and a coupling 105C is formed of material such as CU (copper) in the ⊃-shape. Incidentally, FIG. 13 is an external appearance perspective view of a single body of the yoke 101.
The yoke 101 is firmly mounted on the coil bobbins 103A, 103B by an adhesive so that the first legs 101A, 101B with the adhesive applied thereon are inserted into bobbin holes 103a, 103b of coil bobbins 103A, 103B from their end to a nearly central position. On the other hand, the movable piece 105 is slidably mounted on the coil bobbins 103A, 103B so that the respective second legs 105A, 105B of the movable piece 105 are inserted into the bobbin holes 103a, 103b until they are brought into contact with the first legs 101A, 101B, respectively from the other ends of the bobbin holes 103a, 103b of the coil bobbins 103A, 103B. The movable piece 105 is held in a state where the pair of second legs 105A, 105B of the movable piece 105 have been attracted to the S and N poles generated at the tips of the pair of first legs 101A, 101B of the yoke 101. Incidentally, with the pin of a lever (not shown) inserted into an attaching opening 105D for the movable lever 105, spring force F1 is always applied to the movable piece 105 through this lever.
For this reason, while a current is not passed through the coils 104A, 104B, the movable piece 105 is attracted to the yoke by attractive force F2 by the permanent magnet 102 against the spring force F1. When a current is passed through the coils 104A, 104B to generate a magnetic field opposite to the magnetic flux of the permanent magnet 102, the attracted movable piece 105 is separated from the yoke 101 by the spring force F1. The movement of the movable piece 105 when it is separated provides a necessary operation.
Incidentally, the structure of the self-holding type plunger as described above has been proposed In J-UM publication 5-47450 in addition to FIG. 12.
In the conventional plunger structure described above, the yoke 101 is fixed to the coil bobbins 103A, 103B through the adhesive. Until the adhesive hardens, further operations must wait. This reults in poor operability and long assembly time. In addition, because adhesive is used, working rooms become dirty and, because adhesives tend to have an unpleasent odor, working enviroments become less hospitable due to the smell. Further, when external force is applied to the bobbin before the adhesive hardens, the bobbin has a defective posture when fixed to the yoke which adversely affects operation of the movable piece.